Conventionally in the medical field and the like, various radiation detectors (so called “Flat Panel Detectors”, hereinafter referred to as FPD's) that record radiation images related to subjects when radiation which has passed through the subjects is irradiated thereon have been proposed and are in practical use. Examples of such FPD's include those that utilize semiconductors such as amorphous selenium that generate electrical charges when irradiated with radiation. FPD's of this type that utilize the so called optical readout method and the TFT readout method have been proposed.
Accompanying the spread of radiation imaging apparatuses that employ the FPD's described above, functions in which a series of imaging operations are performed to obtain a plurality of images, and a combined image is generated from the plurality of images (tomosynthesis imaging, longitudinal imaging, energy subtraction imaging, and CT imaging, as disclosed in Patent Documents 2 and 4, for example) are being focused on. In such imaging operations, there are cases in which patients (subjects) move between imaging operations because there are temporal lags among the imaging operations. If body movements of the subjects occur, there is a problem that the combined image cannot be generated accurately.
In addition, in the imaging functions described above, mechanical errors, such as movement errors of radiation sources and FPD's due to installation errors of apparatuses, deterioration of apparatuses over time, etc., may adversely influence the combining of images separately from body movements of the subjects. Hereinafter, these mechanical errors will be described with longitudinal imaging as an example. First, longitudinal imaging is executed in cases that a possible imaging range is smaller than a target to be imaged. In longitudinal imaging, an FPD is moved along a predetermined axis of movement, and receives irradiation of radiation that has passed through a single subject each time the position of the FPD changes. A readout operation is executed to read out signals from the FPD each time radiation is irradiated (each time that a radiation image is recorded), to obtain image data that represent a radiation image by each readout operation. Then, the image data are combined such that they are linked, and image data that represent an elongate part of a subject can be obtained. Such imaging is generally referred to as longitudinal imaging.
When radiation images are combined as described above, there are cases in which there are shifts at the boundaries of the combined image obtained by combining the radiation images due to inclinations of an imaging surface of an FPD. There are several types of inclinations that cause this problem. Inclinations of imaging surfaces will be described in detail below with reference to FIG. 27 and FIG. 28.
First, a of FIG. 27 is a diagram that schematically illustrates an imaging system for obtaining radiation images, viewed from the side thereof. In FIG. 27, reference numeral 100 denotes a radiation source, reference numeral 101 denotes a stand that guides the movement of a rectangular panel shaped FPD 110, and reference numeral 102 denotes an imaging surface of the FPD 110. Note that in the following description, the subject of images which are recorded is a grating 103 in order to facilitate understanding of the problem. That is, radiation 104 emitted from the radiation source 100 passes through the grating 103 is irradiated onto the imaging surface 102 of the FPD 110. In addition, the rectangular panel shaped FPD 110 is set such that the surface of the panel and an edge of the panel are parallel to the direction in which the stand 101 extends (the direction indicated by arrow A), and is moved along the direction indicated by arrow A. In this case, the direction indicated by arrow A is the axis of movement. Prior to and following movement of the FPD 110, the radiation 104 which has passed through the grating 103 is irradiated onto the FPD 110 in a still state, to obtain a first radiation image and a second radiation image.
As one problem, the imaging surface 102 (that is, a two dimensional matrix of pixel sections that constitute the imaging surface) is inclined by an angle α due to assembly error of the FPD 110. In addition, there are cases in which the FPD 110 itself is provided in an inclined manner with respect to the axis of movement, even in the case that the imaging surface 102 is not inclined within the FPD 110, that is, the matrix that constitutes the imaging surface 102 is formed parallel to the surface and an edge of the rectangular panel shaped FPD 110. If the imaging surface 102 is inclined in this manner, the radiation images of the grating 103 which are obtained by a first imaging operation and a second imaging operation will be those as illustrated as b and c of FIG. 27. That is, if the portion in the vicinity of the lower edge of the first recorded image and the portion in the vicinity of the upper edge of the second recorded image are to be linked by template matching or the like, the lengths of the subject at these portions will differ, and shift will be present at the boundary therebetween.
In this case, the angle of inclination α of the imaging surface 102 is an angle of inclination with respect to the axis of movement which extends in the direction indicated by arrow A, because the FPD 110 is set as described above.
Next, another problem will be described with reference to FIG. 28. a of FIG. 28 is a diagram that schematically illustrates an imaging system for obtaining radiation images, viewed from the front thereof. Note that a radiation source is not shown in FIG. 28, but is arranged so as to irradiate radiation along a direction perpendicular to the drawing sheet. As illustrated in FIG. 28, there are cases in which the imaging surface 102 is inclined by an angle γ with respect to an edge of the panel within a plane parallel to the surface of the panel of the FPD (a plane parallel to the drawing sheet) due to an assembly error of the FPD 110. In addition, there are cases in which the FPD 110 itself is provided in an inclined manner with respect to the axis of movement, even in the case that the imaging surface 102 is not inclined within the FPD 110, that is, the matrix that constitutes the imaging surface 102 is formed parallel to the surface and an edge of the rectangular panel shaped FPD 110. Note that in FIG. 28, a portion of the pixel sections are denoted by reference letter G. If the imaging surface 102 is inclined in this manner, the radiation images of the grating 103 which are obtained by a first imaging operation and a second imaging operation will be those as illustrated as b and c of FIG. 28. That is, if the portion in the vicinity of the lower edge of the first recorded image and the portion in the vicinity of the upper edge of the second recorded image are to be linked by template matching or the like, shifts will be generated at the boundary therebetween.
Note that in this case as well, the angle of inclination γ of the imaging surface 102 is an angle of inclination with respect to the axis of movement which extends in the direction indicated by arrow A, because the FPD 110 is set in the same manner as that illustrated in FIG. 27.
In the case that the size of the FPD 110 is 40 cm·40 cm, and the distance (SID) from the radiation source to the imaging surface is 180 cm, the aforementioned shifts at the boundary of the combined image will be approximately 0.5 mm at the end of the image if the angle of inclination α is 0.31 degrees, and approximately 0.5 mm if the angle of inclination γ is 0.07 degrees, which are significant shifts.
Cases in which the inclinations of the imaging surface are constant during movement of the FPD have been described above. However, if the imaging surface gradually becomes inclined accompanying movement of the FPD, the inclination of the imaging surface will change accompanying movement of the FPD. Similar problems will occur in such a case as well. FIG. 29 is a diagram that schematically illustrates such a situation. Note that the case illustrated in FIG. 29 is that in which not only inclination of the imaging surface, but also displacement in the horizontal direction occurs accompanying movement of the FPD 110. Such a phenomenon will occur if the precision of a guide mechanism for guiding the movement of the FPD 110 is low, or if a gap between a guide rod and a guiding member that slides along the guide rod, as constituent elements of the guiding mechanism, is set comparatively large.
In such a case, the radiation images of the grating 103 obtained by a first imaging operation and a second imaging operation will be those illustrated as b and c of FIG. 29. In this case as well, if the portion in the vicinity of the lower edge of the first recorded image and the portion in the vicinity of the upper edge of the second recorded image are to be linked by template matching or the like, shifts will be generated at the boundary therebetween.
Further, the problem of shifts being generated at the boundaries among images are not caused only by inclination of the imaging surface, but also in cases that the imaging surface is displaced from a predetermined position during irradiation of radiation. This displacement will be described in detail below.
FIG. 30 is a diagram that schematically illustrates a situation in which displacement of the imaging surface occurs. a of FIG. 30 is a diagram that schematically illustrates an imaging system for obtaining radiation images, viewed from the side thereof. In the case that longitudinal imaging is to be executed, the FPD 110 is to be positioned at predetermined positions during a first imaging operation and a second imaging operation such that the positions overlap to a certain degree along the direction indicated by arrow A. However, if a mechanism for moving the FPD 110 has undergone changes over time, there are cases in which the FPD 110 will be displaced from the predetermined positions in a direction parallel to the direction indicated by arrow A during each irradiation operation. FIG. 30 illustrates a case in which the FPD is displaced downward for a length Δy from the predetermined position for the second irradiation operation.
In this case, the radiation images of the grating 103 obtained by the first and second imaging operations will be those as illustrated in b of FIG. 30 and c of FIG. 30. In this case, the images are combined such that the position, within the first image indicated by y0 in the drawing and the upper edge of the second image are matched. However, a shift will be present at the boundary between the images, because the upper edge of the second image is displaced for the length Δy.
Further, there are cases in which displacement occurs in a direction perpendicular to the direction indicated by arrow A. FIG. 31 is a diagram that schematically illustrates a situation in which such displacement occurs. a of FIG. 31 is a diagram that schematically illustrates an imaging system for obtaining radiation images, viewed from the front thereof. Note that a radiation source is not shown in FIG. 31, but is arranged so as to irradiate radiation along a direction perpendicular to the drawing sheet.
When longitudinal imaging is executed, the FPD 110 is to be placed at predetermined positions which are aligned in the direction perpendicular to the direction indicated by arrow A during a first irradiation operation and a second irradiation operation. However, if a mechanism for moving the FPD 110 has undergone changes over time, or if the stand 101 (more specifically, rails that guide the movement of the FPD 110) is bent as illustrated in FIG. 31, there are cases in which the FPD 110 will be displaced from the predetermined positions in a direction perpendicular to the direction indicated by arrow A during each irradiation operation. FIG. 31 illustrates a case in which the FPD is displaced rightward for a length Δx from the predetermined position for the second irradiation operation.
In this case, the radiation images of the grating 103 obtained by the first and second imaging operations will be those as illustrated in b of FIG. 31 and c of FIG. 31. In this case, the images are combined such that the first image and the second image are matched in the horizontal direction, that is, the direction perpendicular to the direction indicated by arrow A. However, a shift will be present at the boundary between the images, because the second image is displaced for the length Δx.
For these reasons, a technique for correcting shifts at the boundaries within combined images due to inclination of the imaging surface of the FPD and displacement of the FPD from predetermined positions (hereinafter, referred to as mechanical errors of the imaging surface) has been proposed (refer to Patent Document 1).
Meanwhile, there is a possibility that body movements of the subject will occur during longitudinal imaging utilizing an FPD as described above. In the case that body movements occur, it is not possible to appropriate combine a plurality of radiation images, and accurate measurement is hindered. Therefore, it becomes necessary to execute imaging operations repeatedly. For this reason, techniques for detecting body movements of subjects and ceasing imaging operations or issuing warnings indicating that body movements have occurred have been proposed (refer to Patent Documents 2 and 3).
Note that similar errors due to body movement of subjects during imaging operations and problems due to mechanical errors of imaging apparatuses may also occur during tomosynthesis imaging, in which a plurality of images are obtained while moving an FPD and/or a radiation source.